I get what you are saying, it is unintuitive that you can call a water molecule water. Because water, when found in nature, is always a mixture, right? Needs more than a single substance to be a mixture.
Entertain this, though: a group with a single element is still a group.
Wouldn’t you say a pure water substance can be reduced to a single water molecule? It still is pure water substance.
They will, but you have to follow some protocols, like not having beard where the masks is supposed to seal around your face, not using it more than ~3 times (iirc), not trying to clean it (just let it rest for some days on a clean surface) and etc.
Basically always seal testing.
Also, iirc the N stands for not oil resistant, so any oil staining ruins it. I’d guess that includes sneezing on it.